I. Field of the Invention
The present invention relates generally to balancing systems of the type used in manufacturing facilities.
II. Description of the Prior Art
There are many manufacturing operations which require a heavy load to be lowered from an elevated position and to a lower position in preparation for a manufacturing and/or assembly operation. For example, in the manufacture of automobiles with hoods or rear hatchback doors, it is conventional to utilize a carrier to move the jig fixture centering the closure from an elevated position and to a predetermined lower position where the closure is secured to the vehicle body.
In order to provide a controlled movement of the jig fixture or other load from an elevated position and to a lower position, it has been previously known to utilize one or more pneumatic balancers. Such pneumatic balancers typically have an elongated cable with a free end secured to the load. As the load is lowered from an elevated and to a predetermined lower position, the pneumatic balancers exert an upward force on the load to thereby control and cushion the downward descent of the load as it is moved into its assembly position.
One disadvantage of these previously known balancing systems, however, is that certain vehicle components, such as vehicle hoods or hatchbacks, require a very heavy jig fixture, e.g. 1700 pounds, such that even a pair of pneumatic balancers is inadequate to provide to the jig fixture the vertical compliance required in its lower position to self-locate it onto the vehicle with a low level of constraint transmit to this vehicle. While it is possible to multiply by 2, 3 or 4 the number of these pneumatic balancers in order to increase the upward force exerted on the load during its descent, such pneumatic balancers are relatively expensive and are also subject to periodic maintenance and repair.
The previously known pneumatic balancers conventionally apply a substantially constant force, e.g. 100-300 pounds each, upwardly on the load, e.g. 1800 pounds during the entire descent of the load from its elevated and to its lower position. With two pneumatic balancers, only 30% of the weight can be compensated and the 70% remaining will be supported by the vehicle shell, causing not only some damage on the contact area, but generating also an inaccurate setting of the closure onto its opening.